The cyclins and their corresponding cyclin-dependent kinases are involved in the regulation of the cell cycle. Liver is the model system used for examining the role of cyclins in cell proliferation. After a two-thirds partial hepatectomy, at two hour intervals, between 12-48 hours, regenerating liver was examined for expression of cyclins A and B1 and the associated p34cdc2 kinase mRNA. A sensitive reverse transcriptase/polymerase chain reaction (RT/PCR) assay was developed and a mRNA internal standard was made for cyclins A and B1 and p34cdc2. The ability to quantify the levels of each mRNA present at various times post hepatectomy was possible by the inclusion of an mRNA internal standard. Initially, the levels of all three mRNAs were low but began to rise at 18-20 hours. Cyclin B1 and p34cdc2 kinase mRNAs peaked at 24-26 hours, decreased at 34 hours and increased again by 40 hours. In contrast, cyclin A mRNA expression peaked at 24 hours and remained at that level for up to 48 hours. Furthermore, the temporal and spatial localization of cyclins A and B1 and p34cdc2 kinase proteins was determined by indirect immunofluorescence. During the time period examined, the expression of these three proteins was found only in the hepatocytes. The intracellular localization was initially cytoplasmic (18 hours post hepatectomy) and became nuclear during mitosis (24-26 hours post hepatectomy) and then gradually disappeared. These results establish the expression of cyclins A and B and p34cdc2 kinase during liver regeneration in vivo and will serve as a baseline to compare the expression of these genes under differing biological conditions in vivo and in vitro.